The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Sensors gather information from components of an engine system. The information is received by a control module that controls the engine system based on the received information. For example, a mass air flow (MAF) sensor may measure mass air flow. The MAF sensor may have a square wave output. The frequency of the MAF sensor output may vary relative to the mass air flow to the MAF sensor. The relationship between the frequency of the MAF sensor output signal and mass air flow may be known such that the mass air flow at a particular frequency may be found using a mass air flow vs. frequency look-up table.
The control module uses the measured mass air flow to control fuel injection. It may be useful to know the mass air flow that enters a cylinder between particular cylinder events. A cylinder event may be a cylinder air intake event, and may also be referred to as a low resolution (LORES) event. Some systems determine mass air flow using the average frequency between the engine events. This average frequency is used as an index for the mass air flow vs. frequency look-up table. However, averaging techniques may not account for non-linearity in the relationship between mass air flow and frequency, and thus may result in an inaccurate average mass air flow.
Another way to determine mass air flow between cylinder events involves converting the frequency axis of the mass air flow versus frequency look-up table to a period axis. This conversion may be based on the relationship between frequency (cycles per second) and period (seconds per cycle). Mass air flow may also be converted to mass based on the relationship between mass air flow (mass per second), mass, and period. A timing module may receive the MAF output signal and measure the period of each cycle of the signal. The mass vs. period look-up table may be used by the timing module to determine a mass based on the period. The mass and period may then be accumulated between cylinder events.
The MAF sensor signal may not be synchronized with the cylinder events, such that an error may be associated with an uncounted partial MAF signal cycle between cylinder events. The magnitude of the error may be based on the period of the partial signal compared to the overall time between cylinder events. Vehicle operating conditions may occur where the output of the MAF sensor is at a low frequency (i.e., low mass air flow) and the cylinder events occur frequently (i.e., high RPM). A relatively small number of MAF sensor output signal cycles such as 5 may occur per cylinder event, such that a partial signal may create a potentially large error.
Referring to FIG. 1, a timing diagram illustrating a possible error due to cylinder event and MAF sensor output timing is demonstrated and generally identified at 10. In FIG. 1 there are five complete MAF sensor output pulses per cylinder event when the falling edges of the MAF sensor output line up with the cylinder (LORES) events. However, assuming that mass air flow calculations may be based on falling edges of the MAF sensor 34 output, five pulses occur between the first and second cylinder events and four pulses occur between the second and third cylinder events. This may cause different mass air flow readings for the same overall mass air flow.
Referring now to FIG. 2, a graph 12 illustrates sampling error in an exemplary 8 cylinder engine with varying engine RPM values and varying MAF sensor frequency values. Accuracy of 95 percent or greater may be considered acceptable. The accuracy is based on the percentage of time that a calculation may not yield an error in engine operation. The FIG. 2 shows accuracy falling below the acceptable range at high engine RPM levels and/or low MAF sensor output frequency levels. For example, at a MAF sensor frequency of 1000 and an engine RPM of approximately 3300, the accuracy is approximately 75%.
Referring now to FIG. 3, a graph 14 illustrates sampling error in an exemplary 4 cylinder engine with varying engine RPM values and varying MAF sensor frequency values. Accuracy of 95 percent or greater may be considered acceptable. The graph 14 shows that the accuracy falls below the acceptable range at high engine RPM levels and/or low MAF sensor output frequency levels. For example, at a MAF sensor frequency of 1000 and an engine RPM of approximately 6500, the accuracy is approximately 77%.